Not sure how you ended up with a stereo receiver in your setup instead of a surround receiver with proper bass management and a LFE (subwoofer) output. But, you should list your budget as well as the size of your room and if there are any size restrictions/location restrictions which may come into play.
SVS is always a common brand which is suggested for their high quality at reasonable prices for subwoofers. You will need to run the 'RECORD OUT' lines to the subwoofer, and hopefully the Marantz will send audio through those outputs using the digital input from the TV. I expect it will without issue. All bass management will need to occur within the subwoofer.
For standard stereo listening, the Marantz is fine, but it really limits you in a more standard surround sound setup. You get zero benefit from surround sound in movies and TV viewing, and the lack of bass management isn't a good thing. But, if you are happy, then keep rolling with it I suppose.